Cylinder one misfire code

Diagnosing a misfire takes a bit more effort than the dealer expended. Spark plugs are only part of the path. Did they check the coil on that cylinder, any of the connections (the boot, etc). A competent mechanic can put a scope on this car and see the exact behavior of the cylinder. I would get another mechanic who can properly diagnose problems.

 

If there is a light on, there is a code to be read (or did someone forget to clear the CEL?).

If the only DTC (code) currently thrown is a P0301 (misfire on cylinder #1), one would generally check the plug, coil, or fuel injector. But when that fails there is more to check, and here is the same workup for DTC P0301 that a professional mechanic would use : https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/Repair%20Manual/04pruisr/05/2054m/cip0300.pdf

What kind of injector cleaner did they use; when you add a bottle of something to the gas tank you want a product that contains PEA or Techron. It is doubtful they hooked into the fuel rail to clean, and most don't have an injector flow bench. With 256k, it also could be time for new/rebuilt injectors. I bought four (4) for $23 online when I changed out the plugs last year.

What kind of plugs did the mechanic install ? (example: Denso SK16R11)

Check the coil for "carbon tracking" in the rubber area, if no carbon tracking is found (and after testing of course) you may need to replace the coil. Since you first noticed the problem (or a little before), has it been raining much in your area ?

On a side note, those four (4) plugs likely cost around $50 retail and the labor shouldn't have been over one (1) hour to R/R. Like @jb in NE said above, it might be time to shop around a little.

 

I would have switched the #1 ignition coil with the #2 ignition coil and see if the problem moved to the #2 cylinder.
An ignition coil can appear fine until it gets stressed by heat or load then it can act up.These coils are relatively cheap ebay has some Denso coils for $15 apiece. Hard to understand why the mechanics didn't change out the coil when they replaced the plug. A faulty coil, a faulty spark plug or a clogged injector are usually the causes of this code. Switching the coils is a basic step in troubleshooting this problem, as suggested it well may be time to look at a different mechanic.